1. Field of the Invention
The present invention relates to a hollow stepped shaft which is formed in a region intermediate between its opposite ends or at one of its ends with a stepped portion larger in diameter than its axial portions and which is hollowed about its axis over its entire axial length or except for a portion thereof. The invention relates, inter alia, to a method of forming such a hollow stepped shaft and to a form or product made thereby.
2. Description of the Prior Art
A hollow shaft of this type has so far been formed by a method as described JP 2001-334317 A which uses a hollow tube as its starting blank material. The hollow tube is filled with a filler of a low melting point material and then loaded in an open die or a closed die in which the hollow tube together with the filler is compressed from both its axially opposite sides to cause its medial region to expand in diameter and to deform into an annular recess provided in the die.
The unit cost of a tubular material as the blank amounts in weight unit cost to three to five times higher than that of a solid material (rod stock), however. For this reason, the conventional method using a tubular material as its starting blank has the problem that the material cost is high.
Also, the axial compression of a blank that is already hollow to form a radial expansion as shown in FIG. 22A gives rise to the problem that a further axial compression of the radial expansion to increase its thickness causes a part of its inside to be bent and folded axially as shown in FIG. 22B and creates cracks in the grain flows which may become a critical internal defect.